Process and apparatus using a plurality of catalytic beds in series for the production of gas oils with a low sulphur content

ABSTRACT

A process for hydrotreating gas oils comprises:  
     at least one first step a) for hydrodesulphurization in which said gas oil cut and hydrogen are passed over a catalyst disposed in a fixed bed;  
     at least one subsequent second step b) in which a gas fraction containing at least a portion of the hydrogen sulphide contained in the total effluent from said first step is recovered along with an effluent that is depleted in hydrogen sulphide; and  
     at least a third step c) in which at least a portion of the effluent depleted in hydrogen sulphide from step b) and hydrogen are passed over a catalyst disposed in a fixed bed that is identical to or different from that used in step a).  
     The quantity of catalyst used in the first step is strictly more than 50% by weight of the total quantity of catalyst used in said process.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of fuels for internalcombustion engines. More particularly, it concerns the production of afuel for a compression ignition engine. In this field, the inventionrelates to a process for transforming a gas oil cut to produce a highcetane index fuel that is highly desulphurised. The current legislationin the majority of industrialised countries requires that engines mustcontain less than about 500 parts per million (ppm) of sulphur. Incertain countries, there are currently no standards imposing maximumcontents for aromatic compounds and nitrogen. However, in some countriesor states, for example Sweden and California, and in particular inSweden, certain classes of diesel fuel must already satisfy very strictspecifications. In that country, class II diesel fuel must not containmore than 50 ppm of sulphur and class I fuel must not contain more than10 ppm of sulphur. Currently in Sweden, class III diesel fuel mustcontain less than 500 ppm of sulphur. Similar limits also have to besatisfied for the sale of that type of fuel in California. Newenvironmental standards concerning the storage zone in the refinery forthis type of fuel (the storage zone is termed the “gas oil pool” by theskilled person) for 2005 will necessitate reducing the sulphur contentof gas oils to 50 ppm or even to 30 ppm. Other specifications may alsoconcern the aromatic compound content, cetane index, density or endpoint.

PRIOR ART

[0002] The term “gas oil” as used in the present description designatescuts of this type originating from straight run distillation (SR) ofcrude oil and cuts of this type from different conversion processes, inparticular those from catalytic cracking.

[0003] Hydrodesulphurisation constitutes the essential refining processfor bringing those products to the required sulphur contents.

[0004] Processes for hydrodesulphurising conventional gas oils termedone-step processes have already been proposed as they use a singlecatalytic bed. A summary description of such processes can be found, forexample, in Hydrocarbon Processing, September 1984, page 70 or inUllmann's Encyclopaedia of Industrial Chemistry, Vol. A18, page 65-66.The transformation of hydrocarbons in the reaction zone is then carriedout in the presence of a certain partial pressure of H₂S, essentiallydue to desulphurisation reactions. Now, the presence of H₂S in ahydrotreatment catalyst has the effect of slowing downhydrodesulphurisation reactions very significantly. Such processes weresufficient when the desired sulphur contents in the final product werenot too low (up to 300-500 ppm). For deeper desulphurisation, theinhibiting effect of H₂S becomes critical.

[0005] For that reason, so-called two-step process layouts using twocatalytic beds were proposed. A device for stripping the effluent at theoutlet from the first bed can eliminate the major portion of the H₂S,and the second catalyst bed functions at a lower partial pressure of H₂Swith better desulphurisation activity.

[0006] United States patent U.S. Pat. No. 5,292,428 proposes a processfor hydrotreating hydrocarbon feeds including gas oils, comprising twoor more catalytic zones, with elimination of H₂S at the outlet from thefirst zone and addition of fresh hydrogen to the second reactor. The H2Sthat is formed is generally eliminated using an amine washing unit. Theactivity of the catalyst in the second catalytic zone is then improvedbecause of the lower partial pressure of H₂S. However, to reach highdegrees of desulphurisation, sufficient to satisfy the most strictsulphur specifications (50 ppm or even 30 ppm), it is necessary toemploy severe operating conditions from the start of the cycle byincreasing the operating pressure and temperature and/or by employingsufficiently low HSVs (volume of feed per volume of catalyst per hour).Increasing the temperature at the start of the cycle can prove to bedeleterious as regards the cycle time. The operating pressure can onlybe increased within reasonable limits for reasons of process economy.Finally, for a given unit capacity, operating at a lower HSV means usinga larger volume of catalyst, which involves a supplemental operatingcost. Further, French patent FR-A-2 757 532, for example, also describesa two-step process using a catalyst in the second step containing anoble metal from group VIII, enabling very deep desulphurisation of gasoil cuts. However, that process has a certain disadvantage because ituses a noble metal in the second step, increasing the cost of such acatalyst and secondly, increasing the sensitivity to hydrogen sulphidethe amount of which at the outlet from the first step must be limited tothe maximum if a reasonable service life of the second step catalyst isto be obtained.

[0007] In contrast to the currently unpublished teaching from theApplicant's French patent application, national filing number FR00/02809, which recommends using a quantity of catalyst that is lessthan or equal to half the total quantity of catalyst used in the processin the first step a) of the hydrodesulphurisation process, it hassurprisingly been discovered that in certain specific cases, excellentresults can be obtained by using, in step a) of a process comprising atleast two hydrodesulphurisation steps, a quantity of catalyst in thefirst step that is strictly greater than half the total quantity ofcatalyst employed in that process. This is particularly the case when anold desulphurisation unit is to be modernised that functions with asingle hydrodesulphurisation reactor that cannot satisfy the newspecifications required, in particular concerning the sulphur content inengine fuels authorised for sale. In the event of such modemisation,adding a supplemental hydrodesulphurisation zone can produce enginefuels corresponding to new standards and it is understood that, from thepoint of view of economics, if the reactors that are added havedimensions that are relatively reduced with respect to the singlereactor already in place; the cost of modifying the old unit will bevery attractive compared with constructing a whole new unit or adding areactor with larger dimensions that that of the single reactor alreadypresent.

DESCRIPTION OF THE INVENTION

[0008] Surprisingly, we have discovered a process for producingkerosines and/or gas oils with a very low sulphur content that canimprove the efficiency of the catalyst by acting firstly on the partialpressure of H₂S and by optimising the distribution of the residencetimes (and thus the catalyst volume) in the different catalytic zones.This process can achieve deep desulphurisation with a lower hydrogenconsumption than in prior art processes, which constitutes a furthervery important advantage for the refiner, who is constantly on thelookout for processes that consume small amounts of hydrogen, a preciousasset in the refinery.

[0009] More precisely, in the process of the invention, the hydrocarboncut is typically a kerosine and/or a gas oil, with an initial boilingpoint in the range about 150° C. to 250° C., and with an end point inthe range about 300° C. to 400° C. The process of the invention employstwo hydrodesulphurisation zones each containing at least onehydrodesulphurisation catalyst containing, on a support, at least onenon noble metal from group VIII associated with at least one group VIBmetal.

[0010] In its broadest form, carrying out a process forhydrodesulphurisation of a kerosine and/or gas oil cut in accordancewith the present invention comprises:

[0011] at least one first hydrodesulphurisation step a) in which saidgas oil cut and hydrogen are passed over a catalyst disposed in a fixedbed comprising, on a mineral support, at least one metal or compound ofa metal from group VIB of the periodic table in a quantity, expressed asthe weight of metal with respect to the weight of finished catalyst, ofabout 0.5% to 40%, and at least one non noble metal or compound of a nonnoble metal from group VIII of said periodic table in a quantity,expressed as the weight of metal with respect to the weight of finishedcatalyst, of about 0.1% to 30%;

[0012] b) at least one subsequent second step b) in which a gas fractioncontaining at least a portion of the hydrogen sulphide (H₂S) containedin the total effluent from said first step is recovered along with aneffluent that is depleted in hydrogen sulphide; and

[0013] c) at least one third step c) in which at least a portion of theeffluent that is depleted in hydrogen sulphide from step b) and hydrogenare passed over a catalyst disposed in a fixed bed that is identical toor different from that used in step a) comprising, on a mineral support,at least one metal or compound of a metal from group VIB of the periodictable in a quantity, expressed as the weight of metal with respect tothe weight of finished catalyst, of about 0.5% to 40%, at least one nonnoble metal or compound of a non noble metal from group VIII of saidperiodic table in a quantity, expressed as the weight of metal withrespect to the weight of finished catalyst, of about 0.1% to 30%, saidprocess being characterized in that the quantity of catalyst used in thefirst step a) is strictly greater than 50% by weight of the totalquantity of catalyst used in said process. Preferably, the quantity ofcatalyst used in the first step is about 60% to about 90% of the totalquantity of catalyst used in said process.

[0014] The first step a), deep hydrodesulphurisation, is normallycarried out in a reaction zone comprising at least one fixed catalystbed. This zone can contain a plurality of catalyst beds that may beidentical to or different. Similarly, step c) is normally carried out ina reaction zone comprising at least one fixed catalyst bed. This zonecan contain a plurality of catalyst beds that may be identical to ordifferent. The different catalytic zones can be arranged in differentreactors. A plurality of catalytic zones, with the exception of thefirst zone, can be integrated into one and the same reactor.

[0015] Surprisingly, as will be shown in the comparative examples below,it has been shown that total hydrogen consumption is lower when asmaller quantity of catalyst is used in the second reactor than when thequantity of catalyst is identical in the first and second reactor.

[0016] In order for the catalyst bed or beds in the reaction zone forstep c) to stay in the sulphurised state, the concentration of H₂S atthe inlet to this second catalytic zone is kept at a sufficient level byadjusting the amount of hydrogen sulphide elimination in step b). Stepb) for recovering a liquid feed that is depleted in hydrogen sulphideand a gas fraction containing at least a portion of the hydrogensulphide contained in the total effluent from step a) can be carried outusing any method known to the skilled person. By way of illustration, itis possible to carry out this recovery of a gas fraction containing atleast a portion of the hydrogen sulphide contained in the total effluentfrom step a) by stripping or entraining using at least onehydrogen-containing gas at a pressure that is substantially identical tothat prevailing in the first step and at a temperature of about 100° C.to about 450° C. under conditions for forming a gaseous strippingeffluent containing hydrogen and hydrogen sulphide. This recovery canalso, for example, be carried out by flashing the total effluent fromstep a). In a particular implementation of the invention, the gasfraction recovered in step b) containing hydrogen sulphide is sent to azone for at least partial elimination of the hydrogen sulphide itcontains, from which purified hydrogen is recovered, at least a portionof which is recycled to the inlet to at least one of steps a), b) or c)indifferently. All of the purified hydrogen can be recycled. Complete orpartial recycling of the purified hydrogen can be carried out to just asingle step or to two of the steps, or to all three. In this zone,hydrogen purification from a gas mixture containing hydrogen andhydrogen sulphide originating from the zone for at least partialelimination of hydrogen sulphide, is normally carried out using aconventional technique that is well known to the skilled person and inparticular by a prior treatment of this gas mixture with a solution ofat least one amine under conditions that can eliminate hydrogen sulphideby absorption, said amine usually being selected from the group formedby monoethanolamine, diethanolamine, diglycolamine, diisopropylamine andmethyldiethanolamine. In a particular implementation of this absorption,the gas mixture will be brought into contact with a basic solution,preferably an aqueous solution of an amine selected from the groupmentioned above, which forms an addition compound with hydrogen sulphideto enable the production of purified gas containing proportions ofhydrogen sulphide of far less than 500 ppm by weight and usually lessthan about 100 ppm by weight. Usually, the quantity of remaininghydrogen sulphide is less than about 50 ppm by weight and mostfrequently less than about 10 ppm by weight. This method for purifying agas mixture is a conventional method that is well known to the skilledperson and has been widely described in the literature. As an example,it has been succinctly described for the treatment of natural gascontaining hydrogen sulphide, for example, in Ullmann's Encyclopaedia,volume A12, pages 258 ff. Within the context of the present invention,treatment with an aqueous amine solution is normally carried out at atemperature of about 10° C. to about 100° C. and usually about 20° C. toabout 70° C. Normally, the quantity of amine used is such that the moleratio of hydrogen sulphide to amine is about 0.1:1 to about 1:1 andusually about 0.3:1 to about 0.8:1, for example about 0.5:1. Thepressure at which the hydrogen sulphide is absorbed by the amine isnormally about 0.1 MPa to about 50 MPa, usually about 1 MPa to about 25MPa and usually about 1 MPa to about 10 MPa. The amine solution isconventionally regenerated by varying the pressure. To produce a dryergas and to eliminate more of the hydrogen sulphide initially present inthe gas mixture, it is also possible to provide at least a portion ofthis gas mixture with a complementary treatment such as treating the gasleaving the absorption step, in a zone for adsorbing hydrogen sulphidecomprising at least one reactor and usually at least two adsorptionreactors containing a sieve, for example, preferably a regeneratablesieve or, for example, zinc oxide and operating, for example, at atemperature of about 10° C. to about 400° C., normally at about 10° C.to about 100° C. and usually at about 20° C. to about 50° C. at a totalpressure of about 0.1 MPa to about 50 MPa, normally about 1 MPa to about25 MPa and preferably about 1 MPa to about 10 MPa. In thisimplementation, when the adsorption zone comprises two reactors, onereactor is used to treat the gas while the other is being regenerated orthe material it contains is being replaced to dry and desulphurise thegas mixture entering said zone. At the end of the complementarytreatment, the hydrogen sulphide content of the gas is normally lessthan 1 ppm by weight and usually of the order of a few tens of ppb byweight.

[0017] The operating conditions for step a) normally include atemperature of about 240° C. to about 420° C., a total pressure of about2 MPa to about 20 MPa and an hourly space velocity of liquid feed ofabout 0.1 to about 5, and that of step c) normally comprises atemperature of about 240° C. to about 420° C., a total pressure of about2 MPa to about 20 MPa and an hourly space velocity of liquid feed thatis strictly higher than the hourly space velocity of the liquid feed forstep a).

[0018] The catalyst(s) used in the different catalytic zones arehydrodesulphurisation catalysts. These catalysts can be conventionalcatalysts such as those described in the prior art, for example one ofthose described by the Applicant in French patent applications FR-A-2197 966, FR-A-2 583 813 and in European patent document EP-A-0 297 949.It is also possible to use commercial catalysts such as those sold byPROCATALYSE. These catalysts each comprise at least one metal orcompound of a metal from group VIB and/or at least one non noble metalor compound of a non noble metal from group VIII, on a suitable mineralsupport.

[0019] The catalyst support is generally a porous solid. This support isnormally selected from the group formed by alumina, silica,silica-aluminas, zeolites, magnesia, titanium oxide TiO₂ and mixtures ofat least two of these mineral compounds. Alumina is routinely used. Thesupports for the two catalysts used in steps a) and c) are selectedindependently of each other, but can optionally be of the same nature orbe identical.

[0020] The group VIB metal is normally selected from the group formed bymolybdenum and tungsten, and the group VIII metal is normally selectedfrom the group formed by nickel, cobalt and iron, usually selected fromthe group formed by nickel and cobalt. Combinations such as NiMo andCoMo are typical. In a preferred implementation, the catalyst used instep a) and that used in step c) each comprise molybdenum or a compoundof molybdenum in a quantity, expressed as the weight of metal withrespect to the weight of finished catalyst, of about 2% to 30% and ametal or compound of a metal selected from the group formed by nickeland cobalt in a quantity, expressed as the weight of metal with respectto the weight of finished catalyst, of about 0.5% to 15%. Usually acatalyst comprising nickel as the group VIII metal and molybdenum as thegroup VIB metal is used in step a) and in step c).

[0021] In a preferred implementation, the catalyst used in step a) andthat used in step c) each also comprise at least one element selectedfrom the group formed by silicon, phosphorus and boron or one or morecompounds of this/these element(s).

[0022] In a further implementation, the catalysts used in step a) and instep c) each comprise at least one halogen. Normally, the quantity ofhalogen is about 0.1% to about 15% by weight with respect to the weightof finished catalyst. The halogen is usually selected from the groupformed by chlorine and fluorine and in a particular implementation, thecatalysts used will contain chlorine and fluorine.

[0023] The temperature of the different catalytic zones is preferably inthe range 260° C. to 400° C., more preferably in the range 280° C. to390° C. The operating pressures used are preferably in the range 2 MPato 15 MPa and preferably in the range 2 MPa to 10 MPa.

[0024] The overall hourly space velocity or overall HSV (volume of feedper volume of catalyst per hour) is in the range 0.1 h⁻¹ to 10 h⁻¹.

[0025] When the process comprises two catalytic zones, the distributionof the residence times in the catalytic zones is such that the residencetime in the second catalytic bed is a maximum of 50% of the overallresidence time.

EXAMPLES Example 1 (Comparative)

[0026] A gas oil composed of 75% of straight run, SR, gas oil and 25% ofLCO (light cycle oil, gas oil from catalytic cracking) was treated usinga one-step process with a single bed of catalyst in a reactor. Thecharacteristics of the gas oil are shown in Table 1 below.

[0027] The catalyst used is sold by Procatalyse under the tradereference HR 448. It contains nickel and molybdenum in amounts that liewithin the ranges mentioned for steps a) and c) of the process of theinvention. After activating the catalyst by sulphurisation, the reactorwas kept under a total pressure of 30 bar g (1 bar g is equal to 0.1MPa) and at a temperature of 340° C. A quantity of hydrogencorresponding to a H₂/feed ratio of 270 l/l was injected, the mixture offeed and hydrogen traversing the catalytic bed in upflow mode. The gasoil was injected into the bottom of the reactor at a liquid flow rate of200 cm³/h. TABLE 1 Principal characteristics of gas oil feed Density15/4 (g/cm³) 0.8712 Sulphur (ppm by wt) 13435 Distillation, ASTM D86  5%point 235 50% point 286 95% point 354

[0028] To achieve a sulphur specification of 50 ppm by weight in the gasoil produced, it was necessary to use 500 cm³ of catalyst, giving a HSVof 0.4 h⁻¹, as shown in Table 2. TABLE 2 Hydrogen consumption to achieve50 ppm sulphur specification (Feed: 75% SR gas oil + 25% LCO; 1 reactor;pressure = 30 bar g) Sulphur specification (ppm) 50 Corresponding HDS(%) 99.63 HSV required (h⁻¹) 0.4 Hydrogen consumption 100 (wt %/feed)

Example 2 (Comparative)

[0029] The same gas oil as that described in Example 1 was treated usinga two-step process with a single bed of catalyst (per reactor) in twosuccessive reactors R1 and R2. A stripping apparatus between the twobeds eliminated the H₂S produced in the first bed.

[0030] This time, there were two beds of the same HR 448 catalyst. Thesecond catalyst bed was subjected to the same operating pressure as thefirst bed (3 MPa).

[0031] Each reactor contained 250 cm³ of HR 448 catalyst. The resultsobtained are shown in Table 3 below. TABLE 3 Sulphur and hydrogenconsumption (Feed: 75% SR gas oil + 25% LCO) Operating conditionsCatalyst volume, R1 (cm³) 250 HSV R1 (h⁻¹) 0.8 Sulphur, outlet R1 (ppm)200 Catalyst volume, R2 (cm³) 250 HSV R2 (h⁻¹) 0.8 Sulphur, outlet R2(ppm) 15 Overall HSV (h⁻¹) 0.4 Hydrogen consumption 94 (wt %/feed)

[0032] Under the operating conditions described above and in particularfor the same quantity of overall catalyst as that used in Example 1, thelayout of processes with intermediate H₂S stripping between the twocatalyst beds produced a better hydrodesulphurisation (HDS) performance(15 ppm of sulphur in the gas oil produced, in place of 50 ppm) for ahydrogen consumption that was reduced by 6%.

Example 3 (In Accordance with the Invention)

[0033] The same gas oil feed as described in the preceding examples wastreated using the same process as described in Example 2 and under thesame pressure, temperature and H₂/feed conditions. The volume ofcatalyst in the first reactor was 200 cm³ while that in the secondreactor contained a volume of catalyst of 100 cm³.

[0034] Under these conditions, as shown in the results given in Table 4below, the HSV was 2 h⁻¹ compared with the second catalyst bed containedin the second reactor, and the overall HSV was 0.7 h⁻¹. These conditionssatisfied the 50 ppm sulphur specification, while further reducing thehydrogen consumption (85% as opposed to 94% in Example 2) with a lowerquantity of catalyst and a higher overall HSV. TABLE 4 Sulphur andhydrogen consumption (Feed: 75% SR gas oil + 25% LCO) Operatingconditions Catalyst volume, R1 (cm³) 200 HSV R1 (h⁻¹) 1.0 Sulphur,outlet R1 (ppm) 300 Catalyst volume, R2 (cm³) 100 HSV R2 (h⁻¹) 2.0Sulphur, outlet R2 (ppm) 45 Overall HSV (h⁻¹) 0.7 Hydrogen consumption85 (wt %/feed)

[0035] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples. Also, the preceding specific embodiments are to be construedas merely illustrative, and not limitative of the remainder of thedisclosure in any way whatsoever.

[0036] The entire disclosure of all application, patents andpublications, cited above and below, and of corresponding FrenchApplication No. 01/04.924, are hereby incorporated by reference.

[0037] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A process for hydrodesulphurisation of a kerosine and/or gas oil cutcomprising: at least one first hydrodesulphurisation step a) in whichsaid gas oil cut and hydrogen are passed over a catalyst disposed in afixed bed comprising, on a mineral support, at least one metal orcompound of a metal from group VIB of the periodic table in a quantity,expressed as the weight of metal with respect to the weight of finishedcatalyst, of about 0.5% to 40%, and at least one non noble metal orcompound of a non noble metal from group VIII of said periodic table ina quantity, expressed as the weight of metal with respect to the weightof finished catalyst, of about
 0. 1% to 30%; at least one subsequentsecond step b) in which a gas fraction containing at least a portion ofthe hydrogen sulphide contained in the total effluent from said firststep is recovered along with an effluent that is depleted in hydrogensulphide; and at least one third step c) in which at least a portion ofthe effluent that is depleted in hydrogen sulphide from step b) andhydrogen are passed over a catalyst disposed in a fixed bed that isidentical to or different from that used in step a) comprising, on amineral support, at least one metal or compound of a metal from groupVIB of the periodic table in a quantity, expressed as the weight ofmetal with respect to the weight of finished catalyst, of about 0.5% to40%, at least one non noble metal or compound of a non noble metal fromgroup VIII of said periodic table in a quantity, expressed as the weightof metal with respect to the weight of finished catalyst, of about 0.1%to 30%, said process being characterized in that the quantity ofcatalyst used in the first step is strictly greater than 50% by weightof the total quantity of catalyst used in said process.
 2. A processaccording to claim 1, in which the quantity of catalyst used in thefirst step is about 60% to about 90% of the total quantity of catalystused in said process.
 3. A process according to claim 1 or claim 2, inwhich in step b), recovery of a gas fraction containing at least aportion of the hydrogen sulphide contained in the total effluent fromstep a) is carried out by stripping using at least a hydrogen-containinggas at a pressure that is substantially identical to that prevailing inthe first step and at a temperature of about 100° C. to about 450° C.under conditions for forming a gaseous stripping effluent containinghydrogen and hydrogen sulphide, along with a liquid feed that isdepleted in hydrogen sulphide.
 4. A process according to claim 1 orclaim 2, in which in step b), recovery of a gas fraction containing atleast a portion of the hydrogen sulphide contained in the total effluentfrom step a) is carried out by flashing the total effluent from step a).5. A process according to any one of claims 1 to 4, in which theoperating conditions for step a) comprises a temperature of about 240°C. to about 420° C., a total pressure of about 2 MPa to about 20 MPa andan hourly space velocity of liquid feed of about 0.1 to about 5, andthose of step c) comprise a temperature of about 240° C. to about 420°C., a total pressure of about 2 MPa to about 20 MPa and an hourly spacevelocity of liquid feed that is strictly higher than the hourly spacevelocity of the liquid feed in step a).
 6. A process according to anyone of claims 1 to 5, in which the catalyst used in step a) and thatused in step c) each comprise at least one metal or compound of a metalftom group VIB selected from the group formed by molybdenum and tungstenand at least one metal or compound of a metal from group VIII selectedfrom the group formed by nickel, cobalt and iron.
 7. A process accordingto any one of claims 1 to 6, in which the catalyst used in step a) andthat used in step c) each comprise molybdenum or a compound ofmolybdenum in a quantity, expressed as the weight of metal with respectto the weight of finished catalyst, of about 2% to 30%, and a metal orcompound of a metal selected from the group formed by nickel and cobaltin a quantity, expressed as the weight of metal with respect to theweight of finished catalyst, of about 0.5% to 15%.
 8. A processaccording to any one of claims 1 to 7, in which the catalyst used instep a) and that used in step c) each comprise nickel as the group VIIImetal, and molybdenum as the group VIB metal.
 9. A process according toany one of claims 1 to 8, in which the catalyst used in step a) and thatused in step c) each further comprise at least one element selected fromthe group formed by silicon, phosphorus and boron or one or morecompounds of this element or elements.
 10. A process according to anyone of claims 1 to 9, in which the catalyst support used in step a) andin step c) are selected independently of each other from the groupformed by alumina, silica, silica-aluminas, zeolites, magnesia, titaniumoxide TiO₂ and mixtures of at least two of these mineral compounds. 11.A process according to any one of claims 1 to 10, in which the catalystsused in step a) and in step c) each comprise at least one halogen.
 12. Aprocess according to any one of claims 1 to 11, in which the catalystsused in step a) and in step c) each comprise a quantity of halogen ofabout
 0. 1% to about 15% by weight with respect to the weight offinished catalyst.
 13. A process according to any one of claims 1 to 12,in which the catalysts used in step a) and in step c) each comprise atleast one halogen selected from the group formed by chlorine andfluorine.
 14. A process according to any one of claims 1 to 13, in whichthe catalysts used in step a) and in step c) each comprise chlorine andfluorine.
 15. A process according to any one of claims 1 to 14, in whichthe gas fraction recovered in step b) containing hydrogen sulphide issent to a zone for at least partial elimination of the hydrogen sulphideit contains, from which purified hydrogen is recovered, at least aportion of which is recycled indifferently to the inlet to at least oneof steps a), b) or c).